public class LinkedListQueue<E> implements Queue<E> {
    /**
     * 节点内部类
     */
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }
    private Node head;
    private Node tail;
    private int size;

    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public int getSize(){
        return size;
    }

    @Override
    public boolean isEmpty(){
        return size==0;
    }

    @Override
    public void enqueue(E e){
        if(tail == null){
            tail = new Node(e);
            head = tail;
        }else{
            tail.next = new Node(e);
            tail = tail.next;
        }
        size++;
    }

    @Override
    public E dequeue(){
        if(isEmpty()){
            throw new IllegalArgumentException("Cannot dequeue from an empty queue");
        }
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if (head == null) {
            tail = null;
        }
        size--;
        return retNode.e;
    }

    @Override
    public E getFront(){
        if(isEmpty()){
            throw new IllegalArgumentException("an empty queue");
        }
        return head.e;
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Queue:front ");

        Node cur = head;
        while(cur != null){
            res.append(cur+"->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }

    public static void main(String[] args) {
        LinkedListQueue<Integer> linkedListQueue = new LinkedListQueue<>();
        for (int i = 0; i < 5; i++) {
            linkedListQueue.enqueue(i);
            System.out.println(linkedListQueue);
        }
        linkedListQueue.dequeue();
        System.out.println(linkedListQueue);
    }
}
